1. Field of the Invention
This invention pertains to a diagnostic method for checking the accuracy of a hydrocarbon sensor in a vapor recovery system, such as in a fuel dispensing environment.
2. Description of the Related Art
Vapor recovery equipped fuel dispensers, particularly gasoline dispensers, have been known for quite some time, and have been mandatory in California for a number of years. The primary purpose of using a vapor recovery fuel dispenser is to retrieve or recover the vapors, which would otherwise be emitted to the atmosphere during a fueling operation, particularly for motor vehicles. The vapors of concern are generally those which are contained in the vehicle gas tank. As liquid gasoline is pumped into the tank, the vapor is displaced and forced out through the filler pipe. Other volatile hydrocarbon liquids raise similar issues. In addition to the need to recover vapors, some states, California in particular, are requiring extensive reports about the efficiency with which vapor is recovered and proof that the vapor recovery systems are working as intended.
A traditional vapor recovery apparatus is known as the xe2x80x9cbalancexe2x80x9d system, in which a sheath or boot encircles the liquid fueling spout and connects by tubing back to the fuel reservoir. As the liquid enters the tank, the vapor is forced into the sheath and back toward the fuel reservoir or underground storage tank (UST) where the vapors can be stored or recondensed. Balance systems have numerous drawbacks, including cumbersomeness, difficulty of use, ineffectiveness when seals are poorly made, and slow fueling rates.
As a dramatic step to improve on the balance systems, Gilbarco, Inc., assignee of the present invention, patented an improved vapor recovery system for fuel dispensers, as seen in U.S. Pat. No. 5,040,577, now Reissue Patent No. 35,238 to Pope, which is herein incorporated by reference. The Pope patent discloses a vapor recovery apparatus in which a vapor pump is introduced in the vapor return line and is driven by a variable speed motor. The liquid flow line includes a pulser, conventionally used for generating pulses indicative of the liquid fuel being pumped. This permits computation of the total sale and the display of the volume of liquid dispensed and the cost in a conventional display, such as, for example as shown in U.S. Pat. No. 4,122,524 to McCrory et al. A microprocessor translates the pulses indicative of the liquid flow rate into a desired vapor pump operating rate. The effect is to permit the vapor to be pumped at a rate correlated with the liquid flow rate so that, as liquid is pumped faster, vapor is also pumped faster.
There are three basic embodiments used to control vapor flow during fueling operations. The first embodiment is the use of a constant speed vapor pump during fueling without any sort of control mechanism. The second is the use of a pump driven by a constant speed motor coupled with a controllable valve to extract vapor from the vehicle gas tank. While the speed of the pump is constant, the valve may be adjusted to increase or decrease the flow of vapor. The third is the use of a variable speed motor and pump as described in the Pope patent, which is used without a controllable valve assembly.
Various improvements and refinements have been developed to make vapor recovery systems more efficient and provide a better estimate of the type and rate of vapor recovery. Amongst these improvements are vapor flow meters, such as disclosed in commonly owned copending U.S. patent application Ser. No. 09/408,292. Additionally, the use of hydrocarbon sensors positioned within the vapor recovery line is also known as shown in commonly owned U.S. Pat. No. 5,857,500 and its parent U.S. Pat. No. 5,450,883, which are herein incorporated by reference. As the use of such sensors proliferates in the industry, it is being discovered that these sensors deteriorate with age, or otherwise may have their performance degrade over time. Therefore, there is a need for the ability to test the sensors to determine if they are still functioning properly. Additionally, as states begin to require proof that the vapor recovery systems are functioning properly, the ability to test the vapor recovery system is becoming more important.
The present invention periodically tests a sensor for determining hydrocarbon concentration within a vapor recovery system for proper operation. Specifically, the control system which controls the vapor recovery system within a fuel dispenser, checks the reading on the sensor every fueling transaction at the beginning of the fueling transaction and at a subsequent time during the same fueling transaction. If the two readings are roughly equivalent, the control system determines if this is the appropriate fueling transaction to trigger a more comprehensive diagnostic test of the sensor. If an appropriate number of fueling transactions have occurred since the last full diagnostic test, the sensor checks to see if the last measured value of hydrocarbon concentration is within an expected range. Further, the diagnostics test the readings of the sensor against a flow of pure air, to make sure that the last measured value is greater than that of pure air. Still further, the sensor can test itself by measuring a flow of vapor known to contain hydrocarbons and comparing the resultant reading to an expected value. If any of these diagnostic tests fail, the control system may generate an alarm indicating that the sensor has potentially failed and needs to be serviced or examined further to determine the cause of the failure.